1. Field of the Invention
The present invention relates to a conductive polyvinyl alcohol (hereinafter abbreviated to PVA) fiber having good mechanical properties such as strength and elasticity, and having good heat resistance and conductivity, to a method for producing the fiber, and to a conductive fabric comprising the fiber.
2. Discussion of the Background
A method has been proposed for producing a conductive synthetic fiber which comprises adding conductive filler such as carbon black to a synthetic fiber. Such conductive fibers are widely used in various industrial fields because they are relatively inexpensive and suitable for industrial mass production. For example, they are widely used for charging and discharging brushes in static duplicators. The temperature inside the duplicators becomes high due to the heat in fixation, and the conductive fibers for these applications are desired not to be deformed even when exposed to heat for a long period of time.
Most popular synthetic fibers such as polyester fibers, polyamide fibers, acrylic fibers and melt-spun polyolefin fibers are unsatisfactory in heat resistance and shape stability at high temperatures, and therefore conductive regenerated cellulose fibers are widely used for such applications (e.g., see JP-A 63-249185, JP-A 4-289876, JP-A 4-289877 and JP-B 1-29887). However, conductive cellulose fibers have poor mechanical properties, and therefore can not satisfy the requirement for high-quality properties such as good workability in producing charging brushes and discharging brushes and good durability during long-time service of products.
On the other hand, using PVA fibers having good heat resistance and good mechanical properties as conductive fibers for these applications has been proposed (e.g., see JP-A 52-144422). However, the conductive PVA fibers have problems because they are produced by adding a large amount of conductive filler having a size of 50 μm or so to the spinning solution containing the PVA. The filler may precipitate and deposit in the spinning solution and the stability of the production process is low. The drawing performance of the filler-containing fibers is extremely bad compared to that of filler-free fibers. As a result, even though the fibers can be conductive, their mechanical properties such as strength and elasticity are worsened.
Further, another method of producing conductive PVA fibers that are free from the problems of process capability and quality has been proposed. In this, the mean particle size of the conductive filler such as carbon black to be added to the spinning solution is reduced, and a polyoxyalkylene-type nonionic dispersant is further added to the spinning solution to thereby prevent the filler from precipitating and depositing in the spinning solution (e.g., see JP-A 2002-212829). In this method, the particle size of the conductive filler may be reduced to 1 μm or so, and it is favorable from the viewpoint that the surface area of the particles is increased so as to make the fibers conductive. However, the necessary amount of the filler to obtain the desired conductivity is at least ten % or more, and it is still problematic in that the filler may precipitate in the spinning solution and the drawing performance of the fibers is poor.
The popularity of mobile phones and electronic appliances has increased in recent years. However, various problems due to electromagnetic waves from them have been discussed, for example, their influences on human bodies and on errors of other electronic appliances. A conductive fabric is well used as an electromagnetic wave shield for mobile phones and electronic appliances. In this application, however, the fabric must have higher conductivity, and the above-mentioned conductive filler-introduced fibers can not act as shields. On the other hand, a metal coating layer can be formed on the surface of a fabric of light and flexible synthetic fiber by a vacuum evaporation method, a sputtering method or an electroless plating method. However, the metal film formed by such a method has problems because its physical properties such as abrasion resistance and weather resistance are worsened due to the chemical change thereof during long-time use. Accordingly, it is desired to further improve the metal film-coated fabric. Further, the conductivity treatment according to the method is extremely expensive and the practical use of the method is therefore limited.
Apart from the above-mentioned method of adding conductive filler to the spinning solution or adding it in the step of preparing the spinning solution, a different method has been proposed widely for producing fibers of higher conductivity. For example, a copper compound such as cupric chloride is applied to a polyacrylonitrile fiber so as to be adsorbed by the surface of the fiber, and then it is reduced with a sulfide to thereby form a thin, conductive copper sulfide layer on the surface of the fiber (e.g., see JP-A 57-21570, JP-A 59-108043). The conductive fiber obtained by the method has copper sulfide bonded to its surface, in an amount of from 5 to 15% by mass relative to the fiber, via a copper ion-trapping group such as a cyano group or a mercapto group existing in the surface of the fiber. In addition, the fiber has a thin coating layer on its surface and therefore exhibits high conductivity. However, the fiber exhibits its conductivity only close to the thin surface-coating layer of copper sulfide having a thickness of 100 nm or so, and therefore its durability is poor. In addition, in order to make the surface of the fiber adsorb the desired amount of copper sulfide thereon, high-temperature and long-time treatment is necessary. Further, the above-mentioned cyano group and mercapto group have a good ability to trap monovalent copper ions, and therefore the divalent copper salt must be intentionally reduced into monovalent copper ions for them. These steps are expensive, and the method has various problems in these points.
For solving the problem of improving the conductivity and the durability of the fibers, a method of infiltrating copper sulfide particles into the depth of fibers has been proposed, in which a sulfide dye-containing polymer material is used for fiber formation and copper sulfide is bonded to the polymer via the sulfide dye in the fibers formed (e.g., see JP-A 7-179769). In the Examples of JP-A 7-179769, conductive PVA fibers are proposed. To attain its object, the method indispensably comprises a step of preparing a sulfide dye-containing polymer material and a step of bonding copper sulfide to the sulfide dye-containing polymer material to give a conductive polymer material. However, the method is still problematic in that it requires some wet-heat treatments and is therefore complicated. The PVA fibers will be swollen during the treatments, and even if they can be conductive, their mechanical properties are worsened and, as a result, they can not be formed into fabrics. Still another problem with the method is that a sulfide dye is indispensable for infiltrating copper sulfide particles into the depth of fibers, and which is expensive.
Another method has been proposed for making a polymer material conductive, which has an amido group and a hydroxyl group (e.g., see JP-A 59-132507). The method comprises dipping a shaped article in an aqueous solution of a mixture of a copper salt and a reducing agent having mild sulfidizing ability, at a high temperature for a long period of time to thereby form a conductive copper sulfide layer to the depth of the shaped article. In fact, however, the copper sulfide layer can exist only in the vicinity of the surface of the shaped article, and therefore, the conductivity of the shaped article processed according to the method is low. Specifically, since the copper salt and the sulfidizing reducing agent in the aqueous solution are directly reacted with each other at a high temperature for a long period of time, the formed copper sulfide particles grow large and, as a result, the dispersed particle size inside the shaped article is inevitably large. In this respect, the method is not for internal conductivity generation but rather essentially for surface conductive layer formation. Accordingly, the method has various problems in that not only the conductivity of the product is low but also the durability thereof is poor, and the process cost is high. Given that situation, it is now desired to develop PVA fibers that have good mechanical properties such as good strength and elasticity intrinsic to PVA fibers and additionally have good electroconductivity, and to propose an inexpensive method for producing them.